CSIRO funded researchers have discovered why young rabbits are immune to RHDV1, but not RHDV2.
It seems that young rabbits have a naturally heightened immune system, but RHDV2 is able to shut down the elevated immune state – whereas RHDV1 is not.
For more information, see the Research Updates in Feral Flyer Issue 350 (the newsletter of the Centre for Invasive Species Solutions) or the original research paper by MJ Neave et al..